Atomic force microscopy is based upon the principle of sensing the forces between a sharp probe or tip and the surface to be investigated. The interatomic forces induce the displacement of the tip mounted on the free end of a flexible cantilever.
The principle of piezoresistance to detect the deflection of the AFM cantilever is described in U.S. Pat. No. 5,345,815 assigned to Stanford University. The cantilever is formed of single-crystal silicon which is implanted with a dopant to provide a piezoresistive region along the length of the cantilever. Deflection of the free end of the cantilever produces stress in the cantilever. That stress changes the electrical resistance of the piezoresistive region in proportion to the cantilever's deflection. A resistance-measuring apparatus is coupled to the piezoresistive region to measure its resistance and to generate a signal corresponding to the cantilever's deflection.
AFM systems have applications beyond their original application of imaging the surface of a sample. The principle of atomic force microscopy has been extended to data storage, as described in IBM's U.S. Pat. No. 5,537,372. In that data storage application, the tip is in physical contact with the surface of a data storage medium. The medium has surface marks in the form of bumps and/or depressions or pits that represent machine-readable information or data located in data tracks. The deflection of the cantilever is detected and decoded to read the data. Data can also be written on the medium, if the medium has a heat-deformable surface, by heating the tip when it is in contact with the medium surface to form bumps or depressions on the medium surface. The tip is heated by a laser beam directed to the tip region of the cantilever, as described in the '372 patent. In another approach for heating the tip, as described in Chui et al., "Improved Cantilevers for AFM Thermomechanical Data Storage", Proceedings of Solid-State Sensor and Actuator Workshop, Hilton Head, S.C., Jun. 2-6, 1996, pp. 219-224, a single-crystal silicon cantilever is selectively doped with boron to provide a conductive path to an electrically-resistive region near the tip. The tip is then resistively heated when current is passed through the conductive path.
It is also possible for the AFM data storage system to operate in the manner similar to that described in the '372 patent, with the tip engaging the medium surface but not in direct contact with the medium surface. Instead, the tip engages the surface of the medium and follows the surface topography of marks without physically contacting the surface. The tip is maintained in sufficiently close proximity to the surface of the medium that van der Waals or electrostatic forces are in effect, even though the tip is not in direct contact with the data marks. The van der Waals forces deflect the tip toward the medium until the tip approaches a data pit which causes a decrease in or even a complete removal of the van der Waals forces. The tip follows and detects the surface topography in this manner. This type of AFM data storage system is based on the "attractive mode" of AFM, as described by Martin et al., "Atomic Force Microscope-force Mapping and Profiling on a Sub 100-.ANG. Scale", J. Appl. Phys., Vol. 61, No. 10, 15 May 1987, pp. 4723-4729.
One of the problems with AFM data storage systems is the difficulty of maintaining the tip on the data tracks during reading of data. Unlike conventional magnetic and optical data storage systems, there is no magnetically or optically recorded servo information that can be decoded and used to servo control the positioning of the tip. In AFM data storage systems, such as that described in the '372 patent, the data density can be 100 times that of a conventional CD-ROM system. The individual data features or marks can be as small as 50 nm and the individual data tracks spaced apart only 100 nm. One type of tracking servo in an AFM data storage system is based on contiguous "wobble" marks placed on opposite sides of the data track centerline in a manner analogous to sector servo marks in magnetic recording, as described by Mamin et al., "High-density Data Storage Using Proximal Probe Techniques", IBM J. Res. Develop., Vol. 39, No. 6, November 1995, pp. 687-688. However, the wobble marks are difficult to detect and fabricate with sufficient accuracy, and it is difficult to achieve a tracking error signal which varies linearly with off-track distance. The '372 patent, in FIGS. 3A-3C, describes an AFM data storage system with tracking control based on a twist of the cantilever, the twist being detected by a quadrant cell photodiode that receives laser light reflected from the cantilever.
What is needed is an AFM data storage system with a tracking servo control system that can maintain the tip on track at these extremely small dimensions.